High oral doses of mebendazole were given for a mean period of 23 months to 22 patients with inoperable alveolar or cystic echinococcosis (Echinococcus multilocularis n = 18, E. granulosus n = 4). Clinical, morphological, biochemical and serological findings and plasma mebendazole levels were monitored. Clinical and biochemical improvement or stabilization was observed in 17 patients but the parasitic lesions did not decrease in size in most instances. One patient died shortly after onset of therapy with hemorrhage of esophageal varices. Three patients with alveolar and one with cystic echinococcosis had evidence of progressive disease such as increase of cholestasis, destruction of lumbar vertebrae and growth of an intraperitoneal cyst. The plasma mebendazole levels (4 hr after the morning dose) of the latter 4 patients were 0.09 ± SD 0.02 µmol/l, while in those with clinical stabilization or improvement it was 0.30 ± SD 0.14 µmol/l (P < 0.001). These preliminary data indicate 1) a good clinical response to chemotherapy in most patients despite unchanged size of the parasitic lesions, and 2) a direct correlation of clinical response with plasma mebendazole levels.